Thanks for the quick reply.
I have found in the last few months low fares LHR-JFK (not the other way round) with only a 3 day advance purchase requirement. I have even seen $400 fares with no advance purchase (just a sat night stay).
Something just feels odd with this. Wondering if it is a holiday in the UK next w/e or something.
Will certainly have to wait it out as $900 is too high!